2, University of California, San Diego, La Jolla, California, United States
VOPO4 is a promising cathode material in Li-ion batteries due to its ability to hold multiple Li-ions per redox center. Specifically, the V5+/V4+ and V4+/V3+ redox couples allow for the insertion of two Li-ions into VOPO4, LiVOPO4 and Li2VOPO4. These phases also exhibit polymorphism, with VOPO4 and LiVOPO4 having seven and three different polymorphic phases, respectively. A thorough comparison of the physical and electrochemical properties of these polymorphs will help determine the mechanisms by which these phases form and how they perform electrochemically. This study focuses on the three most promising LiVOPO4 phases: α1 (layered, tetragonal), β (orthorhombic), and ε (triclinic). All phases are synthesized from a single precursor, LiVOPO4-2H2O, reducing the effects of synthesis methods on the physical and electrochemical properties of the products, allowing for more accurate comparisons. The transformations between the different phases are studied using x-ray diffraction and differential scanning calorimetry. The electrochemical performance of each phase is evaluated using galvanostatic discharge-charge between 1.5 to 4.5 V at various current rates. The lithium diffusion coefficients are calculated using cyclic voltammetry and galvanostatic intermittent titration technique.